Flexible circuits and electronic devices that combine plastic substrates with thin layers of semiconductors are an important emerging technology. These flexible circuits and the devices that incorporate them have advantages that are difficult or impossible to achieve using conventional semiconductor processing techniques and materials. Specifically, these circuits are mechanically flexible lightweight, and durable. In addition, flexible circuits have the potential to be mass produced at a relatively low cost. Various fields and applications in which flexible electronics have great potential include displays, solar cells, smart cards, radiofrequency identification cards (RFID), and medical applications. Perhaps the largest market potential for flexible circuits is in the active matrix flat panel display market due to its never-ending demand for lightweight and robust wireless technologies.
Recently, a dry printing transfer printing technique for producing bendable silicon transistors on plastic substrates has been proposed. (See, for example, Menard et al., Appl. Phys. Lett. 86, 93507-1 (2005). This technique uses a poly(dimethylsiloxane) (PDMS) elastomer stamp to lift a thin single crystal silicon layer from a silicon wafer and transport the silicon layer to a plastic substrate. Using this technique, high-temperature processing steps must be performed on the silicon layer before transfer because the elastomer stamp and the plastic substrate will not withstand high processing temperatures. As a result this technique only allows one side of the silicon thin film to be processed. For this reason, this technique is unsuitable for the production of thin film electronics that require front- and backside processing. Such devices include double gate field effect transistors (FETs), back-gate FETs, complimentary metal oxide semiconductor (CMOS) devices having multiple oppositely-facing p-channel and n-channel transistors, double-sided bipolar junction transistors (BJTs) and heterojunction bipolar transistors (HBTs).
One emerging field where double-sided thin film electronics are highly desirable is in the field of three-dimensional (3D) integrated circuits. These 3D integrated circuits are made from stacked layers of semiconductor single crystals having buried transistor structures and vertical interconnects to provide vertically integrated circuits with a high transistor density per volume. Examples of vertically integrated 3D circuits are described in Xue et al., IEEE Transactions on Electron Devices, 50, 601-609 (2003). These 3D devices are made from multilayers of planar devices integrated into silicon device wafers with vertical interconnects providing conductivity in the vertical direction.